Reciprocating compressors typically include one or more pistons that “reciprocate” within closed cylinders. They are commonly used for a wide range of applications that include, but are not limited to, the pressurization and transport of air, natural gas, and other gases and mixtures of gases through systems that are used for gas transmission, distribution, injection, storage, processing, refining, oil production, refrigeration, air separation, utility, and other industrial and commercial processes. Reciprocating, compressors typically draw a fixed mass of gaseous fluid at a relatively lower pressure from a suction pipe and, a fraction of a second later, compress and transfer the fixed mass of fluid into a discharge pipe at a relatively higher pressure.
The intermittent mass transfer within reciprocating compressor systems produces complex time-variant pressure waves, commonly referred to as pulsations. The pulsations are affected by the compressor operating speed, temperature, pressure, and thermodynamic properties of the gaseous fluid, and the geometry and configuration of the reciprocating compressor and the system to which it is connected. For example, a reciprocating compressor cylinder that compresses gas on only one end of its piston, referred to as a single-acting compressor, produces pulsation having a fundamental frequency that is equal to the compressor operating speed. Similarly, a reciprocating compressor cylinder that compresses gas on both ends of its piston, referred to as a double-acting compressor, produces pulsation having a fundamental frequency that is equal to twice the compressor operating speed. In addition, the compressor cylinders and piping systems have individual acoustic natural frequencies that affect the magnitude and frequencies of the combined pulsations throughout the system.
These pressure pulsations travel as waves through an often complex network of connected pipes, pressure vessels, filters, separators, coolers and other system elements. They can travel for many miles until they are attenuated or damped by friction or other means that reduce the dynamic variation of the pressure.
The pulsations may excite system mechanical natural frequencies, cause high vibration, overstress system elements and piping, interfere with meter measurements, and affect compressor thermodynamic performance. These effects can severely compromise the reliability, performance and structural integrity of the reciprocating compressor and its connected system, as well as flow meters and other compressors connected to the system.
Therefore, effective reduction and control of the pressure and flow pulsations generated by reciprocating compressors, both upstream (i.e., the suction side) and downstream (i.e., the discharge side) of the compressor, is necessary for safe and efficient operation. Current technology involves creating a detailed model of the compressor and its system that is used to predict its pulsation behavior at the specified operating conditions, which are often variable. When such modeling predicts pulsations, associated shaking forces, and component stresses that are judged to be beyond safe limits, based on accepted industry guidelines, sound engineering analysis and/or practical experience, it is customary to employ a system of pulsation attenuation elements.
Common pulsation attenuation elements include pulsation bottles (expansion volumes, often containing internal baffles, multiple chambers and choke tubes), external choke tubes, additional pulsation bottles, and fixed orifice plates installed at specific locations in the both the suction and discharge side of each compressor cylinder. These prior art pulsation attenuation devices can be used singly or in combination to dampen the pressure waves and reduce the resulting forces to acceptable levels. These devices typically accomplish pulsation attenuation by adding resistance to the system. This added resistance causes system pressure losses and energy losses both upstream and downstream of the compressor cylinders. The pressure and energy losses typically increase as the frequency of the pulsation increases, and these losses add to the work that must be done by the compressor to move fluid from the suction line to the discharge line.
Of the aforementioned pulsation attenuation elements, fixed orifice plates are one of the most common elements employed. They have the advantages of relatively easy installation and low cost. They may be used at multiple locations throughout the system. The fixed orifices are typically thin metal sheets having a round hole of a specified diameter, located at the center of the flow channel (usually a pipe) cross-section. The orifice diameter is generally 0.5 to 0.7 times the inside diameter of the pipe in which it is installed. However, smaller and larger diameter ratios are sometimes used. The orifice plate is retained between two adjacent pipe flanges that are held together with multiple threaded fasteners and sealed with gaskets to prevent gas leaks. Once the flanges are installed the orifice plates remain fixed in place, and can only be removed or changed by safely stopping the compressor, completely venting all gas to atmospheric pressure, loosening all the threaded fasteners, removing the original orifice plate, installing a new orifice plate with new gaskets, re-assembling and tightening the threaded fasteners, purging the system to remove air, pressurizing the system with gas and restarting the compressor.
In the majority of applications, compressor operating conditions vary with time, with the variables being speed, suction pressure/temperature, discharge pressure/temperature, displacement, effective clearance volume, and even the gas composition. Operating condition variations may be gradual over time, but are more often intermittent, changing frequently to higher or lower levels as dictated by the demands of the application. Some applications, e.g., natural gas transmission and gas storage, have extreme variations in operating conditions over time. In fact, the majority of reciprocating applications require operation over a wide speed range of conditions as well as multiple flow rates that range from very low flows to very high flows.
Fixed orifice plates are effective in reducing pulsations over a narrow compressor operating range, however they cause an associated pressure drop that adds to the work and power consumption required by the compressor. The system pulsation control design is almost always a compromise between pulsation control and pressure drop or power penalty. For example, a very restrictive (low diameter ratio) fixed orifice plate may be required to adequately dampen pulsations at certain operating conditions. However, at other operating conditions, the pulsations might be acceptable with a less restrictive (larger diameter ratio) fixed orifice plate or possibly with no orifice plate at all. In addition, a fixed orifice plate that controls pulsations with a tolerable pressure drop and power penalty at some conditions, may cause excessive damping, pressure drop and power penalty at other conditions.
There are therefore multiple challenges when trying to achieve pulsation control with pulsation bottles and fixed orifice plates. A typical disclaimer by the pulsation control designer states that, “Orifice and choke tube diameters are selected to provide the optimum pulsation dampening and pressure drop over the entire operating range of the unit. Typically, the predicted pressure drop levels for the compressor will range from at or below American Petroleum Institute Specification No. 618 (API 618) allowable levels at normal and low flow conditions to above API 618 allowable levels at high flow conditions. Additionally, the pulsation dampening will be generally good at normal and high flow conditions, but may be marginal to poor at certain frequencies when operating at the minimum flow conditions.”
Although a fixed orifice plate having a specific diameter may be necessary and effective for pulsation control at one set or range of operating conditions, it may be unnecessary, ineffective, and/or the cause of unacceptably high pressure drop and associated power consumption at other ranges of operating conditions. Therefore, it would be advantageous to change one or more fixed orifice plate diameters as operating conditions change.
As noted above, fixed orifice plates are commonly placed between two mating flanges that are held together with multiple threaded studs and nuts and sealed with gaskets to prevent leakage of process gas to the atmosphere. Optionally, they may be permanently welded into the inside of the piping or other flow passage. Accordingly, the downtime, labor and lost production required for changing fixed orifice plates make this alternative impractical. As a result, compressor systems tend to run with higher pressure and power losses or with higher pulsation induced vibration, and associated risk, than would be optimal if the orifice size could be changed when dictated by operating conditions. In many cases the range of operating conditions has to be reduced or limited to restrict the operation of the compressor system.
In light of the above, there is therefore a need for a practical device that can change the effective orifice resistance to maintain acceptable pulsation control with minimal pressure drop and power consumption as operating conditions change. There is also a need for a device and a means that could quickly and easily change the effective diameter (or flow restriction) while the compressor is pressurized and operating. Such a device would enable the optimal and safe control of pulsations, while minimizing power consumption.